1. Technical Field
This invention relates to spark plugs for automotive vehicles, and more particularly to a spark plug having an electrode which includes a tip portion which is highly resistant to wear caused by exposure to lead and oxidation at high temperatures.
2. Discussion
Spark plugs are used in internal combustion engines to ignite fuel in a combustion chamber. The electrodes of a spark plug are subject to intense heat and an extremely corrosive atmosphere generated by the formation of a spark and combustion of the air/fuel mixture. To improve durability and erosion resistance, the spark plug electrode tips must be able to withstand the high temperature and corrosive environment of the internal combustion chamber resulting from the chemical reaction products between air, fuel and fuel additives.
SAEJ312 describes the specification for automotive gasoline used as a fuel in the United States. The gasoline consists of blends of hydrocarbons derived from petroleum: saturates (50-80%), olefins (0-15%), and aromatics (15-40%). Leaded gasoline contains about 0.10 g Pb/gallon fuel (0.026 g Pb/L), and 0.15% sulfur. In unleaded gasoline there is about 0.05 g Pb/gallon, (0.013 g Pb/L), 0.1% sulfur, 0.005 g P/gallon, (0.0013 g P/L). In addition, there are a number of additives incorporated into the fuel for various reasons. For example, tetramethyllead (TML) and tetraethyllead (TEL) are added as antiknock agents. Carboxylic acids (acetic acid), compounds are added as lead extenders. Aromatic amines, phenols are added as antioxidants. Organic bromine, chlorine compounds are added as scavengers and deposit modifiers. Phosphors and boron containing compounds are added to reduce surface ignition, preignition and as engine scavengers. Metal deactivators are added to reduce oxidative deterioration of the fuel by metals, such as Cu, Co, V, Mn, Fe, Cr and Pb. In addition, carboxylic acids, alcohols, amines, sulfonates, phosphoric acid salts of amines, are used as rust-preventing additives.
The mechanism for ignition in an internal combustion engine is very complex and is briefly discussed here. In the gasoline engine, the rising piston compresses the fuel/air mixture, causing increases in pressure and temperature. The spark ignites the fuel-air charge, and the force of the advancing flame front acts against the piston, compressing the unburned fuel-air charge further. Pre-flame combustion reactions occur in the unburned fuel-air mixture. The pinging noise or knock often associated with internal combustion engines is produced when an extremely rapid combustion reaction occurs in the end gas ahead of the advancing flame front. The formation of the preflame reaction products of the gasoline sets the stage for knock. It is believed that the alkyllead additive must first decompose in the combustion chamber to form lead oxide before it can exert its antiknock effect. The antiknock species must be finely dispersed in the combustion chamber so that adequate numbers of collisions of the critical reacting species with the antiknock agent will occur. However, lead oxide deposits can cause problems of valve burning and spark plug fouling. Lead deposits which accumulate on the spark plug insulator cause engine misfiring at high speed due to the relatively high electrical conductivity of the deposit.
The complete combustion of a hydrocarbon fuel with air will produce carbon dioxide (CO.sub.2), water (H.sub.2 O) and nitrogen (N.sub.2). The ratio of air to fuel by weight, 14.5/1, is the chemically correct mixture ratio. When less air is available, some carbon monoxide (CO) and hydrogen (H.sub.2) are found in the products, whereas if excessive air is available some oxygen (O.sub.2) is found in the products. The atmosphere present during the combustion may cause the hot corrosion of electrodes in the spark plug.
The manufacture of copper (Cu) and nickel (Ni) electrodes for spark plugs is a proven art and has been accomplished in a variety of ways. For instance, U.S. Pat. No. 3,803,892 issued Apr. 16, 1974 and entitled "Method of Producing Spark Plug Center Electrode" describes a method of extruding copper and nickel electrodes from a flat plate of the two materials. U.S. Patent No. 3,548,472 issued Dec. 22, 1970 and entitled "Ignition Plug and Method for Manufacturing a Center Electrode for the Same" illustrates a method of cold forming an outer nickel cup shaped sleeve by several steps, inserting a piece of copper wire into the cup and then lightly pressing the two materials together. U.S. Pat. No. 3,857,145 issued Dec. 31, 1974 and entitled "Method of Producing Spark Plug Center Electrode" discloses a process whereby a copper center core is inserted into a nickel member and attached thereto by a collar portion to assure that an electrical flow path is produced.
The spark plug electrodes produced by the methods disclosed above perform in a satisfactory manner for a relatively short period of driving time when used in vehicles that were manufactured prior to the implementation of the clean air act of 1977 in the United States. After 1977, with modifications to engine and fuel, the operating temperature of most vehicle increased. As a result of the changes in the engines and fuels, some of the operating components in engines have been subjected to the corrosive effects of the exhaust gases. After a period of time of operating at higher temperatures in recirculation gases, some corrosion/erosion can occur at the nickel-based center electrode. Once corrosion has taken place, the electrical flow path deteriorates which can result in lower fuel efficiency.
Presently manufactured spark plugs for automotive vehicles typically include an electrode which is manufactured from nickel. The electrode also typically includes a very small tip portion which is welded to the electrode during manufacture of the spark plug. The tip portion can be in the shape of a sphere or a rivet and is comprised typically of platinum (about 90%) and nickel (about 10%).
The problem with such spark plugs having platinum-nickel tip portions is that the platinum is susceptible to attack by lead and the nickel is susceptible to selective oxidation at high temperatures. This limits the life of the spark plug.
There is thus a need for a spark plug having an electrode construction which allows for a long life (for example, 150,000 miles) of operation before the spark plug requires replacing. There is further a need for such a long life spark plug which can be manufactured by present day manufacturing procedures, which is not appreciably more expensive than presently manufactured spark plugs, and which includes an electrode which is highly resistant to attack by lead and selective oxidation at high operating temperatures.